The invention is based on a method for injecting fuel. A method of this kind can be performed using an apparatus known from German Auslegeschrift No. 1 260 865. In this apapratus, a piston is fitted into the fuel line leading to the fuel injection nozzle; the piston has a longitudinal bore provided with a throttle, and the line segments upstream and downstream of the piston are connected by means of this bore. The piston has control edges by means of which a relief line can be made to communicate with the fuel injection line downstream of the piston. Upon a pressure increase in the injection line, the piston is first displaced far enough, because of the throttling effect of the throttle, that the control edge on the piston opens the relief line. Within this period, a pre-injection takes place, which is terminated by the opening up of the relief line. Upon a further supply of fuel into the fuel injection line, the piston is displaced further and the relief line thereby closed again. At the same time, an additional connecting cross section is established between the portion of the fuel injection line upstream of the piston and the portion of the fuel injection line downstream of the piston, so that the primary injection can now take place, with a higher injection rate. This apparatus serves especially to attain smooth combustion by means of controlled, variable injection rates.
By contrast, in internal combustion engines in which the fuel is injected directly into the main combustion chamber and which are designed for a high maximum full-load engine speed, to attain sufficient combustion, it is necessary that high injection pressures be used at low engine speeds. This dictates that the nozzle cross sections be relatively small. Such a realization causes problems at the maximum full-load speed, because at that time short injection times are required as well. Rapid needle closure is also necessary.